Special ships such as drillships should be able to be anchored in a marine work area, and thus require dynamic positioning systems capable of maintaining their positions under the influences of tides, winds, and wave heights. Therefore, these ships are equipped with thrusters capable of generating a propulsive force to control positions of hulls thereof during a change of direction under water.
The thrusters are typically installed at lower portions (underwater) of the hulls. Thus, when a breakdown occurs, the thrusters are not easily repaired or replaced. Furthermore, when the thrusters break down on the sea, the hulls should be removed to the shore at which a dock is present for repair according to circumstances. It is a canister type thruster that is proposed to settle this difficult point in view of operation and to allow the thruster to be repaired or replaced in the marine work area. An example of the canister type thruster is disclosed in Korean Unexamined Patent Application Publication No. 10-2010-0003161 (published on Oct. 31, 2012).
Meanwhile, a canister is a giant structure having a sectional width of about 5 to 6 meters and a height of about 10 meters, and is difficult to handle because various devices mounted on the canister are very heavy. Thus, there are many difficulties in raising/lowering the canister in the hull or separating the canister from the hull for maintenance to install it again.